A hollow blow molded thermoplastic, such as a thermoplastic polyester or a biaxially oriented polyethylene terephthalate resin, "PET", container, commonly used to contain food or beverage, has excellent physical properties, durability, and a wide range of applications. However, when used in hot-fill applications, certain portions of the container which are not subjected to the biaxial orientation during the blow molding process, such as the neck area, seal edge or thread, commonly referred to as the finish area, are found to soften and deform in an undesirable manner because the temperature of the food or beverage fill is often above the glass transition temperature of the plastic. Many methodologies and processes have been tried in attempt to strengthen the finish area of such containers by enhancing the density of the resin and opacifying and whitening the finish area through crystallization of the thermoplastic resin by heat treating the finish area.
Strengthening the finish area of a thermoplastic container greatly increases the thermal stability of the finish area, improves the mechanical rigidity and increases impact resistance, wear resistance, and external pressure resistance of the container. However, a common side effect of such a thermal treatment of the finish area of the container is an undesirable deformation of the finish area, thus leading to problems with capping and sealing the container.
When crystallizing the finish area it is desirable to maintain the dimensional quality of the finish area while optimizing the crystallization rate. Such dimensional stability and the crystallization rate are each dependent upon numerous variables such as the diameter of the finish area, the material used and the energy applied to the finish area. It has also been found that a common negative effect of crystallization is the distortion of the finish area into wavy segments due to a lack of support within the finish area as the material softens during crystallization.
U.S. Pat. No. 4,476,084 provides a solution to the problem of deformation during heat treating by placing a cold die pin or plug into the bore of the finish prior to heating. The cold die pin assists in maintaining the proper size and shape of the finish area during the crystallization process. Others have found similar solutions. See, for example, U.S. Pat. Nos. 4,388,356, 4,379,099, 4,572,811, 4,590,021 and 5,261,545. The use of such cold plugs and dies however have led to problems when speed of manufacture is a priority. Inefficient heating, failure to properly position the plug and deformation of the thermoplastic container due to the weight of the plug have commonly been experienced, thereby leading to a further search for a fast, efficient way of crystallizing and maintaining the dimensional stability of the neck area of the thermoplastic container.
Thus, crystallizing processes balance two distinct criteria: the crystallization rate and the quality of the finish area. The use of a cold die plug to improve the quality of the finish area negatively affects the crystallization rate and if the plug is not used, thus improving the crystallization rate, the finish quality is negatively affected.